CMMG Mk9 9mm Colt mag lower - fie control pins

[sigstroker]

I acquired one of these a few years back, and recently tried to assemble it. The fire control pin holes are too small, but just on one side. Looks like they didn't ream both sides.

Would it be okay to chase the holes with a 5/32 drill bit? I don't know if I even have one. It's not a big deal to just push the pins in and out on one side, but I might have to tap on the pretty good to get them even in the lower.

[G-forceJunkie]

Pins should be .154" but I have mic'ed some as big as .156 I would buy a .156 reamer.

[sigstroker]

These particular pins have fit fine in other lowers.

I'm not sure I want to spend money on a reamer just for this. I have many other lowers that are just fine, and I don't see myself milling a bunch of 80's.

[beanz2]

Send it back to CMMG.

[ar15barrels]

Originally Posted by sigstroker

I acquired one of these a few years back, and recently tried to assemble it. The fire control pin holes are too small, but just on one side. Looks like they didn't ream both sides.

Would it be okay to chase the holes with a 5/32 drill bit?

First, check the pin fit into the receiver from EACH side, not just by pushing the pin through one side and into the other as you may get a false read on the actual problem.
The actual problem could be hole sizing OR hole misalignment OR both.
Sometimes, the pin holes are actually the right size and fit the pin just fine from EACH side, but the holes are not aligned so the pin binds when you try to push it through the 2nd side from either side.
This will happen the same from both sides of the receiver if the holes are the correct diameter but simply misaligned.

If the pin simply will not slip through one side but will slip through the other side when you are only testing one wall of the receiver for pin tit, then the solution is to ream the small hole out to 5/32".

It's best to start the reamer through the side that's already big enough and then let the reamer cut the other side.
This will correct both for diameter and alignment as the reamer will kick over and cause the 2nd hole to point at the first hole while you are reaming.
When doing this, use the reamer to cut the misalignment out of BOTH holes in the same setup.

A reamer is FAR better than a drill bit and this one here will do the job for less than $10:
https://www.ebay.com/itm/134224364023

[sigstroker]

Originally Posted by ar15barrels

First, check the pin fit into the receiver from EACH side, not just by pushing the pin through one side and into the other as you may get a false read on the actual problem.
The actual problem could be hole sizing OR hole misalignment OR both.
Sometimes, the pin holes are actually the right size and fit the pin just fine from EACH side, but the holes are not aligned so the pin binds when you try to push it through the 2nd side from either side.
This will happen the same from both sides of the receiver if the holes are the correct diameter but simply misaligned.

If the pin simply will not slip through one side but will slip through the other side when you are only testing one wall of the receiver for pin tit, then the solution is to ream the small hole out to 5/32".

It's best to start the reamer through the side that's already big enough and then let the reamer cut the other side.
This will correct both for diameter and alignment as the reamer will kick over and cause the 2nd hole to point at the first hole while you are reaming.
When doing this, use the reamer to cut the misalignment out of BOTH holes in the same setup.

A reamer is FAR better than a drill bit and this one here will do the job for less than $10:
https://www.ebay.com/itm/134224364023

Thanks.

I had already done just as you laid out. Pin would not go in from the left side, so I tried it from the right side, and it was tight but it went in without tapping with a hammer or anything like that. It went into the left side hole from the inside until the very gnat's *** of the edge. If it was painted or cerakoted I would assume the paint was the problem, but I think they just didn't run the reamer all the way through and stopped just short. Like maybe .05" short. The hammer would actually function as it is.

Could I chuck that reamer up in a drill and then just crab the chuck and turn it by hand? That would reduce the chances of me mucking things up.

[ar15barrels]

Originally Posted by sigstroker

Thanks.

I had already done just as you laid out. Pin would not go in from the left side, so I tried it from the right side, and it was tight but it went in without tapping with a hammer or anything like that. It went into the left side hole from the inside until the very gnat's *** of the edge. If it was painted or cerakoted I would assume the paint was the problem, but I think they just didn't run the reamer all the way through and stopped just short. Like maybe .05" short. The hammer would actually function as it is.

Could I chuck that reamer up in a drill and then just crab the chuck and turn it by hand? That would reduce the chances of me mucking things up.

If you have a reamer, you will find that the non-cutting end is slightly smaller than the cutting end.
What I do is slip the non cutting end of the reamer through both sides of the lower and then grab the non cutting end with a cordless drill.
Now you turn the drill on and you draw the reamer back through the lower until its cutting both sides.
Then push the reamer back to where you started before stopping the drill.
Then stop the drill and remove the reamer from the drill and the lower.
The holes are now align-reamed to each other.

[sigstroker]

Okay, thanks.